Tryptophan, as an essential amino acid in the human body, can only be taken through diet, with a lower content in the body, and the concentration of tryptophan in adult plasma is about 40-80 μM. About 95% of tryptophan is metabolized through kynurenine pathway[Armstrong, M. D. and Stave, U. Metabolism. 1973, 22, 561-569.]. Metabolites of this pathway have immunosuppressive effects and play a key role in the tumor immune escape process. The earliest discovered tryptophan metabolism enzyme was tryptophan-2,3-dioxygenase (TDO) [Kotake, Y.; Masayama, I. Z. Physiol. Chem. 1936, 243, 237-244.], and later, indoleamine-2,3-dioxygenase 1 (IDO1) was discovered[Higuchi, K.; Hayaishi, O. 1967, Arch. Biochem. Biophys. 120, 397-403]. Both enzymes can metabolize tryptophan and other indole-containing endogenous substances along the kynurenine pathway, and be the rate-limiting enzymes of this metabolic process. In vivo and in vitro, IDO1 or TDO can oxidatively cleave the 2,3-double bond of the indole ring in tryptophan with the help of molecular oxygen so as to form N-formylkynurenine (NFK), which is further oxidized to form metabolites such as kynurenine, picolinic acid, quinolinic acid and the like (Scheme 1).

Indoleamine-2,3-dioxygenase 1 (IDO1, EC 1.13.11.52) is the first enzyme that catalyzes the metabolism of tryptophan along kynurenine pathway in mammals, and is also the rate-limiting enzyme, and IDO1 is one of the causes of tumor immune tolerance and is associated with poor prognosis in cancer patients[Platten, M.; Wick, W.; Van den Eynde, B. J. Cancer Res. 2012, 72, 5435-5440]. IDO1 is highly expressed in various tumor tissues, which leads to local tryptophan depletion and induces T cells to arrest in G1 phase, thereby inhibiting T cell proliferation; and IDO-dependent tryptophan degradation leads to kynurenine levels increased, also induces oxygen free radicals to mediate T cell apoptosis; and when antigen-presenting cells such as macrophages and dendritic cells overexpress IDO, this may enhance local regulatory T cell (Treg)-mediated immunosuppression, and promote the body's peripheral immune tolerance to tumor-specific antigens. In vitro and in vivo studies have shown that IDO1 inhibitors can enhance the role of tumor therapeutic vaccines, therapeutic monoclonal antibodies, chemotherapy and radiotherapy[Godin-Ethier, J.; Hanafi, L.-A.; Piccirillo, C. A.; Lapointe, R. Clin. Cancer Res. 2011, 17, 6985-6991].
IDO1 enhances regulatory T cell-mediated immunosuppression at the site of infection, prompting the body to develop immune tolerance to pathogenic microorganisms, which is closely related to the emergence of antibiotic and antiviral drug resistance. In addition, IDO1 is further closely related to the pathogenesis of various diseases, including autoimmune diseases, Alzheimer's disease, depression, anxiety, cataract and the like. Therefore, IDO1 inhibitors have attracted more and more attention as potential drugs.
Tryptophan-2,3-dioxygenase (TDO) is another important enzyme regulating the metabolism of tryptophan along kynurenine pathway. In recent years, it has become a research hotspot as an inhibitor. Platten et al. confirmed that TDO is highly expressed in human glioma cells. Inhibition by drugs or knockout of TDO can block the release of kynurenine, and on the contrary, knocking out IDO1 does not affect the concentration of kynurenine, and this suggests that TDO is one of the key enzymes for the metabolism of tryptophan in these cells[C. A. Opitz, U. M. Litzenburger, F. Sahm, M. Ott, I. Tritschler, S. Trump, T. Schumacher, L. Jestaedt, D. Schrenk, M. Weller, M. Jugold, G. J. Guillemin, C. L. Miller, C. Lutz, B. Radlwimmer, I. Lehmann, A. von Deimling, W. Wick, M. Platten, Nature. 2011, 478, 197-203.]. In an evaluation study of 104 human tumor cell lines, Van den Eynde et al found that 20 cell lines only express TDO, 17 cell lines only express IDO1, and 16 cell lines express both TDO and IDO1 (malignant glioma, Mesothelioma, head and neck cancer, pancreatic cancer, non-small cell lung cancer, malignant sarcoma, bladder cancer, gallbladder cancer, etc.). That is, IDO1 inhibitors are effective against 32% of tumor cell lines, TDO inhibitors are effective against 35% of tumor cell lines, and IDO1 and TDO dual inhibitors are effective against 51% of tumor cell lines. In theory, dual inhibitors of IDO1 and TDO will further enhance the effectiveness of tumor immunotherapy[L. Pilotte, P. Larrieu, V. Stroobant, D. Colau, E. Dolusic, R. Frederick, E.De Plaen, C. Uyttenhove, J. Wouters, B. Masereel, B. J. Van den Eynde, Proc. Natl. Acad. Sci. USA 2012, 109, 2497-2502.].
Tryptanthrin is an indolquinazoline based alkaloid and exists in blue plants such as Polygonum tinctorium and Isatis tinctoria. Studies have shown that such compounds have antibacterial, antiviral, anti-inflammatory, anti-parasitic, and anti-tumor effects. Qing Yang et al. of Fudan University synthesized a series of tryptanthrin derivatives and tested their inhibitory activity against IDO1. Among them, the fluorine-substituted tryptanthrin derivative (structure is as follows) has good activity, and the IC50 value of recombinant human IDO-1 is 0.534 μM, and this compound can promote the proliferation of T cells, and the surface plasmon resonance (SPR) experiment proves that the compound can be directly bound to IDO1, and the KD value is 46.8 μM [Yang, S.; Li, X.; Hu, F.; Li, Y.; Yang, Y.; Yan, J.; Kuang, C.; Yang, Q. J. Med. Chem. 2013, 56, 8321-8231]. Qing Yang et al. also introduced an aminomethyl substituent at the 3-position of tryptanthrin and found no significant change in the activity of inhibiting IDO1 (WO 2015070766).

Valliante et al. synthesized a series of tryptanthrin and aza-tryptanthrin derivatives and found that these compounds contribute to the production of TNF-α, and some compounds can have effects at a concentration of 5 μM or lower (U.S. Pat. No. 8,193,185B2). However, this patent does not relate to the IDO1 and TDO targets, and the results of the study also indicate that the introduction of nitrogen atom in the benzene ring is not directly related to the activity, and wherein the 8-nitro-1-aza-tryptanthrin did not exhibit activity at concentration up to 20 μM.